Speckle is a phenomenon created with laser light sources, due to the fact that laser light is coherent. Parallels and synchronized wavefronts simultaneously hit the projection surface. When the light hits the surface, it creates constructive and destructive interference. The first category of interference induces an image deterioration that is often visible by human eye and/or by sensors. In addition to a loss of image quality, visual comfort of the viewer may also be affected.
Several techniques are used in order to remove or reduce speckle. In many cases, light coherence reduction techniques are used. For instance, the light hitting the projection surface is provided from various projection angles. Polarized laser light hitting a depolarized film is also used. Otherwise, illumination using various laser wavelengths may also be used.
Another approach consists in using vibration of the projection surface. The resulting systems are complex, expensive, and involve very specific hardware material.
WO2009/077198 describes an optical system comprising a coherent light source and optical elements for directing light from the source to a target. The optical elements include at least one diffusing element arranged to reduce a coherence volume of light from the source and a variable optical property element. A control system controls the variable optical property element such that different speckle patterns are formed over time at the target with a temporal frequency greater than a temporal resolution of an illumination sensor or an eye of an observer so that speckle contrast ratio in the observed illumination is reduced. The variable optical property element may be a deformable mirror with a vibrating thin plate or film. This solution requires modifying the projection system in order to integrate additional components, such as diffusing elements.
WO2007/112259 describes a system and method for reducing or eliminating speckle when using a coherent light source. A refracting device, comprising a birefringent material, is positioned such that the refracting device intercepts the coherent light. The refracting device rotates, thereby causing the ordinary and/or extraordinary beams to move. The human eye integrates the movement of the beams, reducing or eliminating laser speckle. The refracting device may include one or more optical devices formed of a birefringent material. Wave plates, such as a one-half wave plate, may be inserted between optical devices to cause specific patterns to be generated. Multiple optical devices having a different orientation of the horizontal component of the optical axis may also be used to generate other patterns. Furthermore, the refracting device may include an optical device having multiple sections of differing horizontal components of the optical axis. This solution involves a complex and expensive component, the rotating refracting device. Moreover, the integration of such device requires a specific global design.
CN101477234 discloses a piezo-driven optical lens, which comprises a lens body, rails for providing an axial motion path, a piezoelectric element for providing a driving force and an elastic element for providing pre-stress for contacting the piezoelectric element with the rails. The lens body is at least provided with a hollow seat body, a lens barrel which is capable of moving axially and is positioned in the hollow seat, and a lens group fixed on the lens barrel. The piezoelectric element contacts the rails through the elastic element to drive the lens barrel to move linearly along the rails. This solution provides an efficient auto-focus system, but is not adapted to reduce speckle.
WO 9918456 describes a lens with variable focus comprising a chamber filled with a first liquid, a drop of a second liquid being provided on a first surface zone of the chamber wall, wherein the chamber wall is made of an insulating material. The first liquid is conductive, the second liquid insulating. The first and second liquid are immiscible, with different optical indices and substantially of the same density. Means are provided for positioning said drop in inoperative position on said zone, comprising electrical means for applying a voltage stress between the conductive liquid and an electrode arranged on said wall second surface, and centering means for maintaining the centering and controlling the shape of the drop edge while a voltage is being applied by electrowetting. This solution involves complex liquid/oil encapsulated system and electrostatic actuation.
US2009040602 describes a stress-induced polarization converter in the form of a zero power optical window or, alternatively, a single element, positive or negative power optical lens, that is subject to a controlled amount of symmetric, peripheral stress. The stress may be provided by appropriate mechanical, thermal, hydraulic, electromagnetic/piezo, annealing/molding, or other known techniques. The applied symmetric stress will advantageously be trigonal or four-fold, but is not so limited. This solution involves symmetrical stress, which is not suited for speckle reduction.
Thus, there is a need for a novel micro-projection system with reduced speckle having MEMS micro-mirrors and MEMS components in general, that does not present the above mentioned drawbacks, namely the complexity and costs problems caused by using specific configurations with additional components used only for speckle reduction.